THE GEOCHEMISTRY OF THE GEOTHERMAL AND HYDROLOGIC SYSTEM OF THE SEPARATION CREEK DRAINAGE, THREE SISTERS AREA, CENTRAL OREGON CASCADES

Helium isotope, carbon isotope, and general geochemistry data from a geochemical monitoring project, initiated in response to recent and on-going uplift in the Separation Creek drainage area, indicate the presence of a significant magma-driven geothermal system below the area. The center of uplift is located 5 km west of South Sister volcano. The uplift is thought to be caused by a magmatic intrusion at approx. 6.5 km depth. Despite the fact that most samples for this study were collected from dilute, low-temperature springs, the strong magma-driven geochemical signal demonstrates that such features can provide valuable data regarding the geothermal and hydrologic system when stronger surface evidence for geothermal activity is absent.

The data from this study provide new insights into geothermal discharge through the complex hydrologic system of the Separation Creek drainage. Among other things our data show that a) the geothermal system is long-lived and not directly related to the current period of uplift; b) the magma recharge rate of the system is frequent enough to keep helium isotope ratios at upper mantle values; c) geothermal fluids discharge laterally away from the crest through the hydrologic system and interact along their flow paths with aquifer fluids/rocks, indicated by a correlation of decreasing helium and carbon isotopic ratios with distance from the area of uplift; d) A correlation between helium-3 and chloride suggests that the Separation Creek fluids are not unlike those discharging from the closest hot springs (located at least 25 km away from the volcano), but highly diluted with air-saturated groundwater. This indicates that at least part, possibly a major fraction, of the chloride in the geothermal water is magma-derived. Strong evidence for a geothermal system was not found in springs east of the Three Sisters crest. It is not clear, at this time, if the Separation Creek drainage presents a unique situation or if the strong magma-driven signal observed in the dilute cold springs is a common feature of Cascade volcanoes in regions of high annual precipitation.